The Joint European Torus (JET) is the biggest and most powerful magnetic fusion experiment currently operating. Hosted at Culham Centre for Fusion Energy in Oxfordshire, UK for scientists around Europe under the European Fusion Development Agreement (EFDA), JET was the first machine to routinely produce controlled bursts of energy from nuclear fusion. The next stage in magnetic fusion is ITER, a scaled-up version of JET, capable of lengthy pulses with a 500 megawatt output (compared with JET’s 16 megawatts). Scheduled to begin operations in 2020, ITER will be the proving ground that opens the way to prototype fusion power plants. Present-day facilities like JET are now primarily testing systems for ITER to ensure a successful start-up for the EUR 14 billion machine on which so many hopes rest.
Although the hot plasma produced in tokamaks such as JET and ITER is held away from the vessel by a series of magnets around the machine, the wall materials are nevertheless subjected to residual heat and particle fluxes at the edge of the plasma, as well as damage by the continual bombardment of neutrons.
Up until now, carbon has been the armour of choice to protect tokamaks such as JET. Carbon is hard-wearing and interacts well with the deuterium plasma which is used for most fusion physics experiments. However, as ITER’s remit is to rehearse the high-power plasmas needed for electricity generation, it will require a cocktail of both deuterium and tritium (DT) to achieve these performance levels. Unlike deuterium, tritium is a radioactive fuel and carbon has an unfortunate tendency to absorb it in hydrocarbon deposits. So researchers have looked to a new, all-metal materials combination for ITER. Beryllium, a light metal that expertly rebuffs tritium’s advances, is planned for the main wall of the machine. For the ‘divertor’ area at the bottom, where the plasma is exhausted from the fusion furnace and heat loads are greater, tungsten – with its higher melting point of 3422 degrees Celsius – will be employed. In addition, tungsten is much less susceptible to erosion by DT plasmas than carbon. This negates the problem of material escaping from the ‘divertor’ and polluting the plasma core, which must be kept free of impurities for the tokamak to perform efficiently.
JET – the device on which much of ITER’s design is based – is the ideal test-bed for the beryllium-tungsten wall. If this materials mix is a success in JET, it will give fusion researchers extra confidence for its deployment on the larger, next-generation machine. The ‘ITER-like wall’ project began in 2005 and culminated in a 21-month shutdown of the JET device to strip out the interior of the tokamak and install thousands of customised tiles made of the new materials. With 11 European fusion associations and 19 industrial partners contributing, and experts at ITER eager to see the results, there was no margin for error on the EUR30 million Enhanced Performance 2 (EP2) refurbishment.
Preparations
The ITER-like wall project involved removing around 4000 carbon fibre composite (CFC) tiles from the existing wall, cleaning the vacuum vessel and installing 2700 unique beryllium-, tungsten- and CFC-coated tile assemblies. Each tile is extremely sensitive to damage, and any type of flaw in the new wall would seriously affect the machine’s performance during start-up and subsequent experimental campaigns; thus the remote handling techniques employed had to be highly developed and controlled. In total, more than 7000 components were to be replaced in the project, of varying complexity and ranging in weight from 100g to 130kg. In addition, auxiliary equipment – including a new diagnostic conduit system, cameras, cables and vacuum feed-throughs – had to be fitted to allow scientists to monitor the performance of the wall materials.
The Mascot manipulator at work inside JET during the shutdown
The most significant preparation for the retrofit programme was the commissioning of a redesigned secondary boom with extended reach and manoeuvrability to transport components into the JET machine. The remote handling system comprises two articulating booms, mechanical arms that access the tokamak through ports at opposite ends of the vessel and work in tandem. Both are multi-joint devices, allowing them to snake around the ring-shaped vessel to the point where they are required. By making the secondary boom longer, the primary boom working with the ‘Mascot’ master-slave manipulator had less distance to travel to pick up equipment. Bringing tools to Mascot rather than the other way round would bring a significant time saving. Remote handling engineers estimated that this could reduce the shutdown period by 30%.
Five newly-designed and -manufactured boom sections were added to give a total length of 9.4 metres, compared with the original 4.8 metres. Although implemented for the immediate needs of the EP2 shutdown, compatibility was retained for this auxiliary arm, allowing it to be converted into a primary boom that could be mounted with a manipulator if the need arose in the future.
The upgraded boom’s efficiency was enhanced by a ‘task module’; effectively a toolbox with a set of drawers. The task module is accessible from each side for Mascot to pick up tools and components. The use of the task module reduced the need for manned access to load components and tools for this work. The complex logistics of this carousel of components are controlled by Task Module Manager software, which tracks the content of each module’s drawers and shows the location of the tools needed for the current task.
Model showing the positioning of the two booms in the JET vessel
Mascot itself was also improved for the ITER-like wall project. Changes to its basic structure added to its versatility, load carrying capacity and ease of maintenance. It can now be configured with different lifting attachments, such as a 100kg winch, a third arm, or a mobile camera, for example, which was used in the outage to inspect the rear of components during their assembly to avoid having to stop operations and launch a dedicated survey.
As well as the mobile cameras, the team made enhancements to the main in-vessel viewing system including replacing halogen bulbs with LEDs and installing high-resolution cameras.
After the initial development work was completed, the team ran VR simulations of the JET vessel, allowing the engineers to validate the operational scenarios and check their feasibility, feeding back into the design of the EP2 remote systems. Where new and complex techniques were required, physical mock-ups were made to validate the simulation work.
Once the new equipment was assembled in 2009, the team carried out a dress rehearsal for the shutdown within a full-scale practice area. The In-Vessel Training Facility contains a spare section (‘octant’) from the JET vacuum vessel and enables operators to practice remote handling techniques and fine-tune their procedures ahead of the real installation, working on dummy components but using the actual tools themselves.
Meanwhile, the manufacture of the new wall tiles was a significant challenge in itself. Individually designed and numbered to occupy a specific place in the JET ‘jigsaw’, the tiles included a variety of materials combinations: beryllium, beryllium-coated Inconel, beryllium-coated CFC, bulk tungsten and tungsten-coated CFC. Production was a multinational effort, involving Casting Technology International in Rotherham, Atmostat in Paris and Axsys Technologies in Huntsville, Alabama. The choice of beryllium and tungsten posed some engineering conundrums. With lower electrical resistivity than carbon, beryllium would experience higher forces on its tile support structures. The solution was to split each solid beryllium tile into slices of up to 20 pieces and to keep the coating on the beryllium-Inconel tiles as thin as possible. The tungsten-coated CFC tiles that would make up most of the divertor region brought a different problem. Tungsten and CFC expand at different rates when heated, and in the 1000-plus degree Celsius conditions at the bottom of the JET vessel this would lead to the tiles buckling or ‘delaminating’ – splitting off in layers. The Romanian fusion association, MedC, coated the tiles using a technique known as combined magnetron sputtering and ion implantation, bombarding the material with ions to create an extremely dense, pore free 10-25 μm thin layer of tungsten that will not buckle at high temperatures.
JET tiles, shown in 2009 testing, have been subdivided to reduce forces on supports
Rebuilding work begins
By October 2009, the last experiments in JET’s old configuration had concluded and the engineers were ready to swing into action as the machine entered shutdown mode. The refurbished boom had been successfully commissioned the previous February, and both booms were now craned into place and steered through the port doors into the vessel. All eyes were now on the Remote Handling Control Room, adjacent to the JET machine hall, where the engineers began a gruelling schedule of shifts working in the vessel 16 hours a day, seven days a week throughout the shutdown. Four teams of five operators were supported by around 25 other staff, who oversaw logistics and equipment. The operators were guided by task procedures that mapped out every part of the project workflow in minute detail from beginning to end. First, there was the matter of taking apart the carbon wall to create a blank canvas to work on. Then each new tile, weighing between 2-60 kg, was painstakingly lifted and fixed into place, a task that typically took between 12 and 65 minutes to complete. The wall design required the tile fixing bolts to be hidden in many areas, and this required a complex sequence of steps to ensure the tile was installed, fixed and torque-tightened before installation of the next one could begin.
It was not all plain sailing. After the last of the old wall was removed in August 2010, the inside of JET was surveyed in detail for the first time since the early 1990s. Using both photogrammetry and 3D stereo photography techniques, the survey revealed around 100 discrepancies between the actual geometry of the vacuum vessel and the CAD model that had been used to plan the EP2 upgrades. These anomalies were predominantly a legacy of modifications to JET that had been made manually before the full development of the remote handling system. Slight variations between the design and final positions had crept in during the manual positioning of components on the vessel walls. The differences were often no more than 2mm; but the gaps required between the tiles were, in many cases, less than this. The shutdown’s duration therefore had to be extended to allow for the extra work involved in modifying a number of tiles to fit the real dimensions.
Cutaway model of JET remote handling assembly with booms and their enclosures
In April 2011, the last plasma-facing tile was fixed in place and JET’s ITER-like wall was complete. After a final photographic survey of the vessel the following month revealed no inconsistencies, the shutdown came to an end and the lengthy process of restarting the tokamak began. This culminated in the first plasma in the new JET on 24 August, as scientists from around Europe assembled in the packed machine control room to observe how the wall and the plasma would interact. The experiment was a resounding success: a 15-second 1 MA pulse, compared with the brief flash of light that would normally appear after a long shutdown.
Experiments with the ITER-like wall began in late 2011. Until neutral beam heating power became fully available in early 2012, the main focus has been fuel retention and the migration of tungsten and beryllium at moderate heating power and plasma currents. Experiments are designed to match those with the carbon wall as closely as possible so the effect of the materials can be clearly quantified. Later this year the project engineers plan to push up heating power and study high energy confinement plasmas (H-modes). The challenges here will be to learn how to keep the power exhausted from the plasma compatible with the beryllium and tungsten materials.
JET currently plans to operate with deuterium-only plasmas to the middle of 2012, then shut down to remove tiles and special samples for surface analysis. It will operate again in 2013, again with deuterium fuel. The project team has delayed tritium operation because much of the physics can be explored in deuterium plasmas and the fusion reaction rate is 100 times less, so the activation of the machine by fusion neutrons is much lower. Also, because tritium is a radioactive gas, it makes access to the machine and handling of samples more restricted in a full DT phase. However, in 2014/5 the team plans to run a campaign with tritium.
Since the first flash on 24 August, plasmas have confirmed that deuterium, beryllium and tungsten are all getting along even better than expected. This could spell good news for the ITER project. ITER’s plan is to start off with a carbon wall in its commissioning phase before switching to beryllium-tungsten armour for full-power tritium operations. However, so positive are the early results from JET’s ITER-like wall that the possibility has been mooted of missing out ITER’s carbon stage altogether, potentially saving the project hundreds of millions of Euros.
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More information on JET and ITER is available on www.efda.org, www.ccfe.ac.uk and www.iter.org
This article was published in the February 2012 issue of Nuclear Engineering International magazine.
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